1. Field of the Invention
The invention relates to the application of optical signal processing to mode locked lasers for precision sensing, sampling and spectroscopy.
2. Description of Related Art
Mode locked lasers and frequency comb lasers have provided for advancements in spectroscopy and precision sensing. A mode locked laser was recently combined with a conventional Fourier transform spectrometer (FTS) to obtain an improved signal/noise ratio for spectral absorption measurements (J. Mandon et al., ‘Fourier transform spectroscopy with a laser frequency comb’, in Nature Photonics, vol. 3, pp. 99-102, 2009) and N. Picque' et al., International Patent Application, Publication WO 2010/010437. The use of a frequency comb laser for spectroscopy was suggested by Haensch et al. in U.S. Pat. No. 7,203,402.
As discussed in '435, ‘in the state of the art, frequency comb lasers can be understood as constituting a sub-class of mode locked lasers. Both mode locked lasers and frequency comb lasers produce a train of output pulses at a certain repetition rate frep with a corresponding output frequency spectrum, which can be characterized as a line spectrum with individual frequency linesf=fceo+mfrep,where m is an integer and fceo is the carrier envelope offset frequency. The integer m is also referred to as the comb line order. However, in contrast to mode locked lasers, frequency comb lasers require precise control of the repetition rate and carrier envelope offset frequency.
Indeed, a difficulty limiting the widespread use of frequency comb lasers is precision optical phase-locking of the individual comb lines to at least two external reference frequencies in order to obtain a stable frequency comb. However, at least for optical metrology, frequency measurements can be performed without stabilization of individual comb lines by the use of a modelocked laser as a transfer oscillator (J. Stenger et al., Phys. Rev. Lett., vol. 7, pp. 073601-1-073601-4, (2002)). Using modelocked lasers as transfer oscillators, frequency ratios or frequency differences between two reference frequencies located in widely separated regions of the optical spectrum (limited only by the spectral extent of the spectral coverage of the modelocked lasers) can be precisely measured. This technique, as applied to the measurement of difference frequencies when using a cw laser (instead of a mode locked laser) as a transfer laser is well known in metrology (C. O. Weiss et al., in ‘Frequency measurement and control, advanced techniques and future trends’, vol. 79, pp. 215-247 (2000). The cw transfer oscillator is sometimes also referred to as reference oscillator.
As also shown in '435, cw reference lasers can be used to effectively stabilize the differences between the carrier envelope offset frequencies of two modelocked lasers. This information can then be used for spectral calibration of a FTS for spectral absorption measurements constructed from the two modelocked lasers with a resolution limit corresponding approximately to the repetition rate of the modelocked lasers. As described in '435, such dual modelocked lasers are referred to as coherent dual scanning lasers or CDSLs. Moreover, CDSLs based on high repetition rates allow for high scan rates which are beneficial for rapid signal acquisition. CDSLs based on fiber supercontinuum sources further allow very broad spectral coverage for an FTS and other applications.
More generally, two narrow linewidth cw reference lasers can be used to track the difference between the carrier envelope offset frequencies and repetition rates of two mode locked lasers, as disclosed by P. Giacarri et al., ‘Referencing of the beating spectra of frequency combs’ (International Patent Application, Publication WO 2009/000079) without the need for carrier envelope offset frequency control. However, when applied to Fourier transform absorption spectroscopy, the resolution of this scheme is also limited to the repetition rate of the mode locked lasers, assuming for example that a reference laser is located in between two comb lines and the absolute frequency of the reference laser or the absolute order m of the comb lines is not known. As a result, relatively low repetition rate lasers are implemented which leads to slow data acquisition rates.
In another scheme the beat signal between two comb lines from two separate comb lasers can be directly measured via the implementation of a cw transfer oscillator. Repetition rate fluctuations between the two comb lasers can then be recorded and these recorded repetition rate fluctuations can then be used to simultaneously correct an interferogram between the two comb lasers via implementing a new sampling grid with equidistant optical path length differences (G. Guelachvili et al., World patent application, WO 2010/010444). However, this scheme ideally also uses measurements of the carrier-envelope offset frequencies of the two comb lasers or alternatively implements a second cw laser with a different frequency to which two other comb lines are locked.
Hertz-level resolution in a FTS has been achieved using two frequency comb lasers which are phase locked to two cw lasers which were in turn locked to two high finesse reference cavities as discussed in I. Coddington et al., ‘Coherent multiheterodyne spectroscopy using stabilized comb sources’, Phys. Rev. Lett., vol. 100, pp. 013902 (2008); henceforth ‘Coddington’. However, such a scheme requires at least 4 phase locked loops for locking of the frequency comb lasers to the two cw optical clock lasers, plus additional phase locked loops for stabilization of the cw lasers to the reference cavities. Moreover, the achieved Hz level resolution is generally not required in real-world optical spectroscopy, where Doppler broadened absorption lines of line width Δν≈5×10−7 νx at a frequency of νx are typically encountered. For example, in the visible spectral region Δν≈300 MHz.
A further need exists for a simple FTS scheme based on CDSLs which allows high scan rates as well as high spectral resolution. Moreover, there is still a need for a laser based FTS scheme that can measure emission as well as absorption spectra.